Coupling between Na+, sugar, and water transport across the intestine

Water is absorbed across the small intestine in the absence of external driving forces. However, it has been established that water transport is secondary to active sodium transport. In the upper intestine both sodium and water absorption are largely dependent on the presence of D-glucose. The link between active sodium transport and glucose is the coupled transport of sodium and glucose across the brush border membrane of enterocytes by the Na+/glucose cotransporter (SGLT1). Na+ that enters the cells with glucose is pumped out towards the blood by 3Na+/2K+ pumps on the basolateral membrane, and glucose passes out across the basolateral membrane by facilitated diffusion, the net result being that glucose and sodium are transported across the epithelium. The coupling between Na+, glucose, and water transport is less well understood. It is commonly thought that Na+ transport increases the local osmotic pressure in the lateral intercellular spaces, and that this in turn generates osmotic water flow across the epithelium. Recent work suggests a more direct link between Na+, glucose, and water transport; that is, water is cotransported along with Na+ and sugar through SGLT1. Here we review the evidence for Na+/glucose/water cotransport.